The Labour-led cabinet’s budget passed due to a “pairing” arrangement that saw an opposition councillor miss the vote to make up for Riley’s absence.
Riley remains the councillor for Chepstow’s Bulwark and Thornwell ward but will sit as an independent.
Her resignation from the Labour group has left the party with 20 of the 46 seats.
She attended the council’s annual general meeting in May where she voted as part of the Labour group for Mary Ann Brocklesby to continue as council leader – which was approved by just one vote – but resigned the following week.
“I have done everything I can to address what I feel is unfairness and bullying and after a year of trying to get it addressed I have decided enough is enough.”
She added: “People are saying this isn’t the forum for these complaints, but yet there is no forum. The only thing you can do is leave, and then when people ask you why you’ve left, be honest and tell them why.”
Riley, who was diagnosed with a stage four cancer, external three years ago, said she told her party she had appointments at Cardiff’s Velindre cancer hospital that could only take place on Thursdays, meaning they would clash with monthly full council meetings.
She said she had even “pulled over beside the A465 to get the laptop out on the way home from Velindre” to take part in meetings remotely.
Riley said she “enjoyed” the type of work she does for her community and is in “a good position to be able to do it.”
She added: “I was a very fit and healthy person four years ago. I didn’t really plan for cancer and I did feel that I could make a difference [being a councillor] and I did do a lot of work.”
Garrett said he was not aware of any formal complaint made against him but said he asked Riley “if she could change her appointment” as Labour’s numbers on the council meant it needed every vote.
Brocklesby said: “As a group and as Welsh Labour, we do expect councillors to turn up on key votes and when there are reasons they can’t do that and we’ve had more than one occasion with councillors with serious illness or dealing with serious illnesses within family, we find ways to deal with it and will continue to do so.
“We are very sad Councillor Riley has chosen to leave the party but we respect her decision.”
Welsh Labour has been asked to comment.
